My boss recently added a second microwave oven in the break room, and now the two ovens are tripping a 20-amp breaker whenever they are on at the same time for more than a few seconds. So, yesterday my boss came out and spark-opened (see below) a panel to put a 30-amp breaker on that particular circuit, but I talked him out of doing that since I have always heard 30 amps need #10 wire. A few minutes later, however, my boss came back out of his office and showed me a book of some kind with a chart saying #12 wire is good for 30 amps in residential structures ... but he ended up saying he would wait and ask the electricians coming out to do some other work in a few days.

So again: Is #12 wire ever good for 30 amps in a commercial building?

Note: I say he "spark-opened" a panel because one of the screws for the cover had been pinching a wire against the inside of the panel and that combination made quite a pop when it shorted while he was removing one of the screws! I believe that panel was made by GE.

There are not two code books only one, sure there are some rules that only pertain to residential, but to answer your question, # 12 AWG is good for 25 amps, BUT... 240.4(D) will only allow it at 20 amps. Also, your boss is looking at the 90 degree column, (310.16) thats where he is getting the 30 amps from.

Here in my house, I have all 15-amp breakers even on circuits with #12 wire, and I only use schedule 40 pipe even for drains. All considered, material costs and broad safety margins are an inexpensive combination when compared to the costs potentially incurred through fire and flood.

Tell your boss he is a fool, and that he should not try and read the code book if he has no clue about what is he looking at. He cannot twist things to make them work in his favor.
Ask him what he thinks about 210.21 if he is into reading the code book. Tell him the information he needs is NEVER in just one spot!

Here in my house, I have all 15-amp breakers even on circuits with #12 wire,

Click to expand...

WHY??? That is a simple waste of 5 potential amps.
I hope you do not think you are being any safer, or saving amps for the "future upgrades".
My house is wired with all #12 also, and the only place I have 15A breakers is on my truck shelf.

This answer is based on the 2005 NEC:Table 310.16, the table used to determine the current-carrying capability of copper or aluminum conductors in cables, conduit, or directly buried in earth is very often used incorrectly. No. 12 awg, cu conductors under the 60 degree C and 75 degree C columns have an "ampacity" (current carrying capacity under specific conditions of operation) of 25 amperes when installed under the conditions stated in the title of the table 310.16. No. 12 awg, cu under the 90 degree C has an ampacity of 30 amperes. HOWEVER, under column one, there are asterisks which are explained at the bottom of the table which direct you to a specific section of the NEC Section 240.4(D). Section 240.4(D) states in part: that No. 14 awg, Cu shall not be protected at more than 15 amperes. No. 12 awg, Cu at not more than 20 amperes, and No.10 awg, Cu at not more than 30 A.
There are installations such as motor branch-circuits, HVAC circuits, and welding machine circuits where the above mentioned conductors are permitted to be utilized at higher amperages than those listed above.
Another important factor to be considered is Section 110.14(C)(1), temperature ratings of terminals to which the conductors are connected.
In summary, increasing the overcurrent protection to 30 amperes will cause the circuit breaker terminals to overheat, overheat the conductors, and could be a fire hazard. Finally, should a fire occur, it may even void the fire insurance coverage. DON'T DO IT!! Run another circuit!